Field of the Invention
The present invention relates to an impedance conversion circuit of a condenser microphone using an electron tube (vacuum tube).
Description of the Related Art
A condenser microphone generates an audio signal based on change of an electrostatic capacitance between a diaphragm and a fixed electrode facing each other.
That is, the condenser microphones in which the diaphragm is arranged to face the fixed electrode have an electrostatic capacitance of about several tens of pF and extremely high output impedance, and are thus configured to take out the audio signal through an impedance conversion circuit.
An impedance conversion circuit with a field effect transistor (FET) or an electron tube (vacuum tube) is used for the condenser microphones. Especially, a product of a condenser microphone is provided for studio sound collection using the electron tube for the impedance conversion circuit to improve sound quality.
Among the impedance conversion circuits using the electron tube, there are a current amplifier circuit called a grounded-anode circuit or a cathode-follower circuit, and a grounded-cathode voltage amplifier circuit.
Typically, it is known that a tone color of the microphone with the cathode-follower as the impedance conversion circuit has a tone color close to the condenser microphone using the field effect transistor.
Meanwhile, the condenser microphone using the above-described voltage amplifier circuit for the impedance conversion circuit has a distinctive tone color mentioned “typical vacuum tube sound”, which is different from that of the cathode follower circuit, and this tone color has firm popularity.
The former cathode follower circuit can achieve a wide dynamic range of output, that is, the range between the lowest level limited by residual noise and the largest level limited by distortion.
However, in the latter voltage amplifier circuit, a signal level where distortion occurs is lower than the cathode follower circuit case, and thus a maximum allowable input sound pressure level as a microphone decreases. Therefore, securing of a wide dynamic range is desired when using the voltage amplifier circuit with the vacuum tube as the impedance conversion circuit of a condenser microphone.
Therefore, to expand the dynamic range in the impedance conversion circuit using the voltage amplifier circuit, means for applying negative feedback is employable. That is, a signal source is an electrostatic capacitance (capacitor), and thus a capacitor is connected from a plate to a grid of an inverting amplifier using the electron tube, so that a signal generated on the plate can be fed back to the grid. This is called PG (plate-grid) feedback, and an impedance conversion circuit to which the PG feedback is applied is disclosed in JP 3890301 B2, JP 4426902 B2, and the like.
By the way, according to the configurations of the PG feedback circuit disclosed in JP 3890301 B2 and JP 4426902 B2, even if the feedback amount is tried to be increased to expand the dynamic range, the plate of the electron tube that configures the voltage amplifier circuit has high impedance, and thus has limitations to increase the amount of the feedback.
Therefore, the applicant of the present invention has previously proposed an impedance conversion circuit that can provide a larger feedback amount to an electron tube that configures a voltage amplifier circuit.
FIG. 3 illustrates the configuration of the impedance conversion circuit previously proposed by the applicant. In this example, a second electron tube T2 is provided which configures a PK (plate-cathode) division circuit that receives a plate output of a first voltage amplifier tube denoted by T1. Then, a feedback signal is provided through a capacitor Ck from a cathode of the second electron tube T2 to the grid of the first electron tube T1.
Note that, in the example illustrated in FIG. 3, the entire configuration except the configuration of the feedback circuit with the capacitor Ck is the same as an embodiment according to the invention described below. Therefore, detailed description of the entire circuit will be described below with reference to FIG. 1.
According to the impedance conversion circuit illustrated in FIG. 3, output impedance of the cathode of the second electron tube T2 is lower than output impedance of the plate of the first electron tube T1. Therefore, a negative feedback amount to the grid of the first electron tube T1 can be increased. Accordingly, this impedance conversion circuit allows the voltage amplifier circuit to achieve a wider dynamic range, and to improve total harmonic distortion characteristics against an input level.